Solid state scan converter utilizing electron guns



Nov. 17, 1970 FROM WRITE GUN FROM WRITE GUN I NVERTI NG AMPLIFIER SOLID STATE SCAN CONVERTER UTILIZING ELECTRON GUNS "D Fw Filed Oct. 4, 1968 I7 I9 I8 23 FIG./

OUTPUT FROM READ GUN FIG. 2

26 READ 3 OUTPUT FROM WRITE GUN in- 26b FROM READ GUN MI? READ OUTPUT FIG. 5

INVENTOR GEORGE R. PRUETT FIG. 4

ATTORNEY United States Patent Olfice 3,541,383 Patented Nov. 17, 1970 U5. Cl. 315- 7 Claims ABSTRACT OF THE DHSCLOSURE A solid state target is disposed between a pair of opposed electron guns. The target is constructed from a sheet of semiconductor material having a plurality of discrete areas of a different conductivity type disposed on one side of the material. The write electron gun is scanned across the semiconductor sheet in a preselected scan pattern, with the intensity of the electron beam being modulated according to input information. The electrical charge densities of the discrete areas on the target are changed due to the scanning by the write electron beam. The read electron beam then scans across the discrete areas on the target in a different scan pattern to generate electrical signals representative of the input information by recharging the discrete areas to their original electrical charge densities. The energy level of the write electron beam is substantially greater than the energy level of the read electron beam to provide gain to the scan conversion system.

This invention relates to scan conversion, and more particularly to the use of a solid state image storage device in a scan conversion system.

It is often desirable to convert an electronic scan format to a different type scan format. For example, it may be necessary to change a circular scan format to a horizontal television-type scan format. One type of scan converter which has been heretofore commonly utilized includes an image storage target constructed from a thin copper mesh with materials such as calcium fluoride or zinc sulfide ap plied to portions of the copper mesh. The target is disposed within an evacuated tube between a pair of opposed electron guns, with one electron gun utilized to write an image upon the target by varying the electrical charge density of areas of the target. The second electron gun is then scanned across the target with a different scan pattern and a grid collects electrons which are reflected by the target in order to provide an indication of the image according to the different scan pattern. Because of interaction between the opposed electron beams which pass through the mesh target, problems have arisen with the use of such scan converters due to the introduction of spurious crosstalk signals into the converter output.

In an effort to eliminate such cross-talk problems, scan converters have been developed which utilize photon coupling through a scan converter target. Such converters have utilized a cathode ray tube in combination with a fiber optics faceplate which transmits the cathode ray tube image to a vidicon tube. However, such fiber optics faceplates are prohibitively expensive for use in a majority of applications. Other photon coupling systems have utilized cathode ray tubes coupled to a television camera, but such systems have not been able to provide satisfactory resolution for many applications.

Another scan converting system is described in US. Pat. No. 3,011,089, entitled Solid State Light Sensitive Storage Device, issued to F. W. Reynolds on Nov. 28, 1961. This patent discloses a solid state target having an array of discrete p-n junctions which are selectively capacitively charged by the impingement thereon of a light image on one side and an electron beam on the opposite side. Although in theory such solid state targets provide a number of advantages over the other previously described devices, a practical scan converter has not been heretofore developed utilizing such a solid state target due to deficiencies in resolution and output gain.

In accordance with the present invention, a solid state semiconductor target surface is scanned with an electron beam in a first scan pattern while the intensity of the beam is modulated according to input information. The electrical charge densities of a plurality of discrete areas of an opposed target surface are varied in response to the first electron beam. The opposed target surface is then scanned with a second electron beam having a relatively constant energy level according to a different scan pattern. The variance of the electrical charge densities of the discrete areas of the target surface provide a scan converted representation of the input information.

In a specific aspect of the invention, the target is constructed from n-type semiconductor material with a plurality of discrete areas of p-type semiconductor material within one surface of the target. A write electron beam source is provided with a relatively high energy level which is modulated according to input information. The read electron beam source is provided with a relatively constant low energy level to provide gain to the system.

For a more complete understanding of the invention and for further objects and advantages thereof, reference is now made to the following description when read in conjunction with the figures on the accompanying drawing, in which:

FIG. 1 is a somewhat diagrammatic view of a scan converter utilizing the present target;

FIG. 2 is a diagrammatic cross section of the target utilized in the system shown in FIG. 1;

FIG. 3 is a view of the rear portion of the target shown in FIG. 2; and

FIGS. 4 and 5 illustrate the theory of operation of the present target device.

Referring to FIG. 1, a scan converter designated generally by the numeral 10 comprises a write electron gun 12 which includes a focus coil 14 and a deflection yoke 6. Collimators 17 are disposed within a vacuumized glass tube 18 in order to properly focus the beam of electrons from gun 12. The present target and storage device 19 is disposed midway between the write gun 12 and a read electron gun 20. A focus coil 21 allows focusing of the electron beam from the read gun 20, while a deflection yoke 22 provides a desired scanning motion to the electron beam.

A pair of collimators 23 assist in properly focusing the read electron beam upon the target 19. It will be seen that the general configuration of the present scan converter 10 is somewhat similar to previous Wire mesh target scan converters, with the exception of the target device 19 which eliminates the necessity for write and read collector electrodes and the like. A relatively high voltage is applied to the write electron gun 12, while a much lower 'voltage is applied to the read electron gun 20. Examples of suitable read and write electron guns are the guns utilized in scan converters manufactured and sold as Model H-ll61 and H-1203 by the Hughes Aircraft Company of Los .Angeles, Calif.

Referring to FIG. 2, a relatively thin body 24 of semiconductor material is preferably made from a wafer of n-type silicon. A plurality of discrete areas 26 are defined in the side of the semiconductor body 24' which receives the beam from the read electron gun. Discrete areas 26 preferably comprise relatively heavily doped p-type diffusion areas somewhat similar to diffused regions in an MOS device. As shown in FIG. 3, the discrete areas 26 are symmetrically spaced across the face of the target 19. The size and spacing of the discrete areas 26 will be dependent upon the desired resolution of the target 19. The device 19 may be fabricated according to a number of processes well known in the art. For example, oxide may be grown over one side of a polished silicon wafer. Holes are then etched through the oxide layer and the p-type discrete areas are diffused into the silicon wafer. Alternatively, a plurality of p-type bodies may be grown on the surface of a silicon wafer. Target 19 will normally be extremely thin, with the preferred embodiment having a thickness in the micron range.

A small load resistor 27 is connected in series with the cathode of the write gun 12 to sense the current in the write gun 12. The signal voltage across resistor 27 is fed to an inverting amplifier 28, the output of which is applied to an adder circuit 29. An output sensor and amplifier read output 30 is connected to the semiconductor body 24 in order to sense scan converted electrical indications of the electron beam pattern received by the device 19. The output of sensor 30 is fed to the adder circuit 29. When the device 19 is used in a scan converter, the scanning operation of the read gun 20 is synchronized with the output sensor 30 to provide scan conversion.

Basically, in operation of the scan converter shown in FIG. 2, the write and read electron guns scan opposite surfaces of the target 19 according to different scan patterns. In some instances, the write electron gun 12 will be scanning the target 19 while the read electron gun 20 is at its minimum level. In these instances, the large number of excess electrons supplied by the high intensity write electron gun may override the output signal received by the sensor 30. This represents noise in the output signal from the scan converter.

In order to eliminate the noise due to these excess electrons supplied by the write gun, a voltage is produced across the resistor 27 which is representative of the write electron gun signal. This voltage is amplified and inverted, and then added to the output of the sensor 30. Due to the inversion of the voltage, any noise generated by the write electron gun is thus subtracted from the scan converter output.

FIGS. 4 and 5 diagrammatically illustrate the operation theory of the target 19. Initially, as shown in FIG. 4, a beam of electrons 31 from the read electron gun 20 is scanned across the back face of the device 19 according to a preselected scan pattern. The negative charges of the electrons from the beam 31 reverse biases each of the discrete areas of p-type material to form depletion layers designated generally by the numerals 32a and 32b. Electron beam 31 is scanned over the complete back face of the device 19 with a constant, low energy level such that each of the discrete areas 26 is charged to a reference negative charge with respect to the n-type semiconductor body 24.

The plurality of p-n junctions provided in the device 19 may be seen to provide a plurality of semiconductor diodes whose capacitance may be varied by the impingement of a beam of electrons thereon. After each of the discrete areas 26 have been reverse biased by the read gun 20, the device 19 is in condition to store an image or pattern transmitted from the write gun 12.

As shown in FIG. 5, an electron beam 34 is scanned across the front face of the device 19 according to a preselected scan pattern. The energy level of beam 34 is relatively high and is modulated by input information in order that the desired image is beamed upon the target 19 after one complete scan of the target.

In response to the impingement of the electron beam, the n-type semiconductor body 24 generates hole-electron pairs in a manner shown diagrammatically in FIG. 5. Due to the reference reverse bias previously applied to the discrete areas 26, the holes thus generated diffuse throughout the n-type semiconductor body and are collected by the discrete areas 26. The electrical charges of the discrete areas 26 are then varied from a reverse biased condition to a more positive charge condition. The

target 19 should have a thickness such that the electrons from the Write gun will not penetrate directly to the discrete areas 26, but the target should be thin enough that the generated hole carriers may easily diffuse to the p-type regions.

The magnitudes of the positive charges imparted to the discrete areas are dependent upon the energy level of the electron beam 34, and thus any desired amount of tone gradation may be achieved. For instance, as shown in FIG. 5, discrete area 26a is shown as being restored to a substantially positive charge condition by the impingement of the beam 34. However, the discrete area 26b has not been subjected to the beam 34, and thus remains in the original reversed biased reference condition. Other discrete areas 26 will be subjected to different energy levels by the beam 34 and will thus have electrical charges different than the discrete areas 26a and 26b.

After the write gun beam 34 has been scanned across the front face of the target 19, the read gun 20 again scans the back side of the target 19, as shown in FIG. 4, in order to provide sensing of the image transmitted from the write gun. When the beam 31 from the read gun impinges upon a discrete area 26 whose electrical charge has been varied by the collection of hole carriers, the read gun beam 31 again reverse biases the discrete area 26 back to the reference level. This recharging of the n-p junction diode by the read gun beam 31 generates an alternating current signal in the n-type semiconductor body 24 which is sensed by the output sensor 30. Any noise caused by the write electron beam is subtracted in the manner previously described.

The signal current sensed by the output sensor 30 is representative of the image or pattern transmitted by the write gun 12. For instance, when the read gun beam 31 impinges upon a discrete area 26 which has been substantially uncharged by the hole-electron pairs generated by the impingement of a high energy level beam from the write gun, a substantial amount of charging by the read gun beam will be required to again reverse bias the discrete area. Thus, a relatively large signal current will be generated. However, when the read gun beam impinges upon a discrete area 2 6 which the write gun beam has not impinged upon, little or no charging of the discrete area occurs, and thus a relatively low or no alternating current output is generated.

The scanning of the read gun beam 31 is synchronized with the sensing output 30, in order that the output sensor 30 will provide a representation of the transmitted image according to the scanning pattern of the read gun. In this manner, scan conversion between the write gun 12 and the read gun 20 is accomplished.

The write gun 12 emits electron energy in the range of l5 kev. In a practical embodiment of the invention, one electron-hole pair is generated in the target for each 3-5 ev. electron energy. Thus, approximately 1000 electron-hole pairs are generated for each 5 kev. electron 1mp1ng1ng upon the present target. This large number of electron-holes results in the large gain provided by the present system.

It will be understood that the present invention prov des substantial advantages over previous scan conversron systems exemplified by the disclosure of US. Pat. No. 3,011,089. Substantial gain improvement is provided with the utilization of two opposed electron guns, and improvements in resolution are provided due to the narrow electron beams. Cross-talk between the read and write guns is substantially eliminated. The use of rela tively high voltages for the write gun provides gains in signal current from the write to read guns.

While specific embodiments of the present invention have been described in detail, it will be understood that various changes and modifications may be suggested to one skilled in the art, and it is desired to encompass such changes and modifications as fall within the scope of the appended claims.

What is claimed is: 1. The method of scan conversion consisting of the steps:

(a) scanning an integral semiconductor target surface with a first electron beam in a first scan pattern While modulating the intensity of the beam according to input information,

(b) varying the electrical charges of a plurality of discrete areas of an opposed target surface in response to said first electron beam,

(c) scanning said opposed target surface with a second electron beam having a constant energy level according to a second scan pattern,

(d) sensing the variance of the electrical charges of said discrete areas to provide a scan converted electrical representation of said input information, and

(e) subtracting a portion of said electrical representation to eliminate noise generated by said first electron beam.

2. A scan conversion system comprising:

(a) a first electron beam modulated according to input information,

(b) a semiconductor sheet having a first surface for receiving said first electron beam,

(c) means for scanning said first electron beam across said first surface in a preselected scan pattern,

(d) a plurality of discrete areas of a different type semiconductor material disposed within a second opposed surface of said semiconductor sheet, the electrical charge distribution of said discrete areas being varied in dependence upon said first electron beam,

(e) a second generally constant level electron beam directed toward said second surface,

(f) means for scanning said second electron beam across said second surface according to a different scan pattern, thereby to charge said discrete areas to a reference electrical charge,

(g) means for sensing variances in the electrical charge of said discrete areas to provide electrical representations of said input information, and

(h) means for subtracting a portion of said electrical representations to eliminate noise generated by said first electron beam.

3. The method of producing scan conversion gain consisting of the following steps:

(a) scanning an integral semiconductor target surface with a first electron beam in a first scan pattern while modulating the intensity of the beam according to input information,

(b) varying the electrical charges of a plurality of discrete areas of an opposed target surface in response to said first electron beam,

(0) scanning said opposed target surface with a second electron beam according to a second scan pattern, said second electron beam having a constant energy level which is of significantly lower energy level than said first electron beam, and

(d) sensing the variance of the electrical charges of said discrete areas to provide a scan converted electrical representation of said input information, said electrical representation having substantial gain over said input information.

4. The method according to claim 3, further including the step of subtracting a portion of said electrical representation to eliminate noise generated by said first electron beam.

5. A scan conversion system for producing gain, comprising:

(a) a first electron beam modulated according to input information,

(b) a semiconductor sheet having a first surface for receiving said first electron beam,

(c) means for scanning said first electron beam across said first surface in a preselected scan pattern,

(d) a plurality of discrete areas of a different type semiconductor material disposed on a second opposed surface of said semiconductor sheet, the electrical charge distribution of said discrete areas being varied in dependence upon said first electron beam,

(e) a second generally constant level electron beam directed toward said second surface, said second electron beam having a substantially lower energy level than said first electron beam,

(f) means for scanning said second electron beam across said second surface according to a different scan pattern to charge said discrete areas to a reference electrical charge, and

(g) means for sensing variances in the electrical charge of said discrete areas to provide electrical representations of said input information, said electrical representations possessing substantial gain over said input information.

6. The scan conversion system according to claim 5, further including means for subtracting a portion of said electrical representations to eliminate noise generated by said first electron beam.

7. The scan conversion system according to claim 5 wherein said first electron beam has an energy level in the range of 3 to 5 kev.

References Cited UNITED STATES PATENTS 3,440,477 4/1969 Crowell et a1. 3l5-l1 RODNEY D. BENNETT, JR., Primary Examiner J. G. BAXTER, Assistant Examiner US. Cl. X..R. 

